U.S. Pat. No. 3,909,588 issued to Walker et al. discloses an electric fluid heater using electrodes immersed in an electrically-insulated flow-through tank with controls sensing both fluid temperature and heating electrode current.
U.S. Pat. No. 4,337,388 issued to July discloses a rapid-response water heating and delivery system incorporating water heating means, water temperature sensing means, and proportional integral derivative (PID) method of closed loop control.
U.S. Pat. No. 4,638,147 issued to Dytch et al. discloses a microprocessor controlled flow-through water heater regulating heating power by switching combinations of heating elements of different wattages.
U.S. Pat. No. 4,829,159 issued to Braun et al. discloses a method of switching electrical heating elements loads to reduce switching transients by energizing all loads neither switched off nor full on in sequence.
U.S. Pat. No. 4,920,252 issued to Yoshino discloses a temperature control method for a plurality of heating elements by allocating a required actuating time within one cycle of a predetermined length of time.
U.S. Pat. No. 5,216,743 issued to Seitz discloses a thermoplastic heat exchanger for a flow-through instantaneous fluid heater including a control system using temperature comparisons.
U.S. Pat. No. 5,479,558 issued to White, Jr. et al. discloses a flow-through tankless water heater with a flow-responsive control means.
U.S. Pat. No. 5,504,306 issued to Russell et al. discloses a tankless water heater system incorporating a microprocessor based control sensing water outlet temperature, accepting an option remote temperature-setting means and providing control of heating elements by applying power in fractions of a power line cycle.
U.S. Pat. No. 5,866,880 issued to Seitz et al. discloses using a plurality of heating elements wherein each of the elements receives a substantially equal amount of power and the delay between each element being powered is no more than 32 half cycles.
Electric flow-through fluid heaters, which are often described as electric tankless fluid heaters, heat fluids as they pass through the heat exchanger. The objective of such heaters is to heat fluid as it enters and passes through the heat exchanger to the desired setpoint by the time it is dispensed at the outlet of the heater. In concept, this process is relatively simple to achieve in closed loop systems in which the operating parameters for flow and temperature can be predetermined.
In this type of application, control of the heater may theoretically be accomplished with standard Proportional Derivative Control (PID) algorithms. Additionally, since many applications are commercial or industrial, one is conventionally not limited by the lack of availability of sufficient electrical service for these applications.
In residential water heating, however, one is presented with an entirely different set of conditions. The process of heating fluids in a flow-through heater can be quite dynamic and requires very responsive and precise control of temperature--not only for the user's comfort but also for safety. Over the years, many efforts have been made to design the "perfect" residential "flow-through" or "tankless" water heater. These efforts have been plagued with a myriad of problems relating to the use of conventional flow detection devices that were unreliable and often failed early due to exposure to highly diverse and aggressive water conditions. A number of other problems arise in fluid heaters as described hereinbelow.
Problem 1. The standard method of temperature control in a fluid heater attempts to regulate the output temperature of the fluid, Tout, based on a reference temperature. In many cases, the reference input is a constant temperature called the setpoint, Tsp. In a single-input (inlet fluid at temperature Tin), single-output (outlet fluid) system, the heater attempts to maintain the temperature of the output fluid equal to the setpoint, or Tout=Tsp. In a heater application, it is assumed that Tin is less than Tout. Flow measurement can provide "feed forward" information to facilitate control. Conventional flow measurement devices such as turbines are expensive and can be adversely affected by water conditions. The classical feedback method of achieving this regulation is to first measure the outlet temperature and compute an error as the difference ERROR=(Tsp-Tout). Using some control scheme, such as PID (Proportional Integral Derivative) the system attempts to reduce the error to zero so that ERROR=0 for steady-state fluid flow.
Several problems plague standard control schemes when they are applied for temperature control of fluid in a tank. First, there is a delay between the application of heat and the sensing of a change in temperature in the fluid. Secondly, such systems are typically very sensitive to changes in the components of the system or the presence of noise in the measurements of temperature, causing errors in the measurement of the temperatures that are used by the system. For example, if accurately calibrated thermistors are used initially to measure temperatures, they can change characteristics with age causing the measurements to be in error. Finally, because of system lag time, these systems are generally not effective in controlling the outlet fluid temperature in the presence of disturbances, such as a rapid change in fluid flow rate.
Problem 2. The Seitz et al U.S. Pat. No. 5,216,743 addressed the use of temperature differential to detect flow/no flow conditions and developed a means for not only detecting these conditions but also monitoring temperature gradients by periodically heating the water to maintain low-energy use in standby conditions. A major drawback of Seitz et al's teaching is in the selection of temperature sensing devices, i.e. thermistors. Seitz et al refer to the use of commodity-type thermistors which inherently vary greatly in their resistance one to another and therefore the resulting temperature measurements vary. The result of this variation impacts the temperature measurements which are used to provide responsive control of shutdown of power at "no flow," as well as start up "in flow," and the ability to establish the small temperature gradients necessary for maintaining the standby condition.
Even when the fluid temperature is the same in the heater, the resulting temperature reading obtained from these types of thermistors can differ one to another, for example, by as much as 5 degrees Fahrenheit or more. In most tankless water heaters, differential temperatures measured between two thermistors are used for temperature regulation. Because of the variations in characteristics, such as resistance versus temperature, between one thermistor and another, an inaccurate difference in temperature readings generally exists. In order to compensate for these variations, the control parameters usually include temperature thresholds for temperature measurements. These temperature thresholds are often required to be much greater than desirable. These higher thresholds, coupled with the difference in characteristics between the thermistors, result in widely different performance between one fluid heater and another. In a heater where the deviation in accurate temperature measurement between thermistors is small, the shutdown is more precise and faster, thereby reducing temperature overshoot at shutdown. The response to flow conditions is also shorter, thereby avoiding an obvious delay in providing initial hot water. The energy required to maintain smaller temperature gradients in standby is also reduced.
When the heater's thermistors resistance and resulting temperature measurements vary substantially, as will often be the case with commodity type thermistors, the reverse is true. Then the heater's response to shutdown is longer overheating the water, and the response to flow is longer delaying the start of fluid heating thus delaying the delivery of initial hot water. In standby conditions, the heater will be maintaining an artificial higher threshold to maintain the required temperature gradient and thus be using more energy to heat the water.
One partial solution to these problems might be to individually and manually attempt to match the thermistors with nearly identical characteristics. This cost of this process reduces the benefits in cost of using commodity-type thermistors, particularly in large volumes. Another approach would be to use high-quality thermistors whose characteristics have very close manufacturing tolerances, but these types of thermistors can be quite costly.
Problem 3. Most patented technology directed to control methods for activating the heating elements for flow-through water heaters teach some form of modulation of power to the heating elements. For whole house "flow-through" fluid-heating applications, large wattage heating elements, i.e., 5,500-7,000 watts, are required.
During conditions of variation in fluid flow rate, it is not at all unusual for the modulation of power to a plurality of large wattage elements to vary by 5-10%. These variations in power are inherent in flow-through residential water heaters. In such applications, pressure changes are frequent, particularly in rural homes where water is supplied by a private water well. In the private water well system, as the pressure in the pressure tank drops, the water pump starts in and increases the pressure to maintain the desired water pressure. The desired water pressure is manually set by setting the pressure switch points in the water system's pressure regulator. Changes in pressure as the water pump is cycling result in changes in the flow rate of the water. The changes in water flow cause the flow-through water heater's control system to modulate the power to the heating elements, in wide swings, attempting to track the demand required by the flow changes. In many cases, the home will be served by an older or undersized electric power transformer. The addition of the modulated water-heating load will result in significant variations in line voltage. These types of electric power conditions are quite common in older homes, rural homes, and manufactured housing.
As power to the heating elements is modulated, the voltage variations occur at frequencies that are particularly disturbing, causing "flicker" in the lighting circuits. U.S. Pat. No. 4,829,159 to Braun recognizes the importance of reducing flicker. A method is disclosed for controlling maximum power drawn by the total number of activated heating elements in an oven by independently varying the duty cycles of each individual element. The heating elements are differently sized, with some heating elements being of a much higher wattage than other heating elements. Braun uses different wattage heating elements and "gently" turns related heating elements "on" and "off" within time periods that would not perform in a satisfactory manner for a flow-through water heater application. In order to minimize flicker, the heating elements are controlled to obtain sequential activation (current consumption) of each of a plurality of heating elements independently of one another in an interlinked fashion such that when one load (heating element) is switched off, the next load (heating element of a different wattage) is switched on. Braun uses elements of different wattages and individually differs the duty cycle of each, so that he can obtain an average desired power level while maintaining interlinking of loads during the switching of power to such loads.
Seitz et al in U.S. Pat. No. 5,866,880 teach flicker control through a unique "power sharing" method of control. Seitz' control system adds substantially the same amount of power sequentially to each element by activating a first heating element and then a second within not more than 32 half cycles. Seitz teaches that the switching load from the modulation of a plurality of heating elements should be limited to the maximum coincident load added or removed from the fluid heater's total load. This coincident load is limited to the incremental load change resulting from an individual heating element's power being sequentially added or removed at a single ac half cycle, to or from the total existing load of all the elements.
Problem 4. In fluid heating systems, it is desirable to minimize energy usage during a "standby" condition when there is no fluid flow. This requires accurate temperature measurement. In a flow-through heater, the ability to detect changes in temperature typically requires several seconds. This is because of the time constants for thermistor response combined with delays in other system components. This is particularly true when one is using commodity type thermistors such as the type offered by General Automotive number 25502 as used by Seitz et al and referred to in U.S. Pat. No. 5,216,743. As described in Problem 2 hereinabove, thermistor characteristics, one thermistor to another, vary substantially for these and most thermistors.
Seitz et al's U.S. Pat. No. 5,216,743 teaches a method for maintaining a temperature gradient to be used to control the energy consumed during "standby" conditions. Unless the control method takes into consideration the need to pulse energy in very small increments over a period of several seconds to allow the system to actually begin to detect and respond to very small, i.e. one (1) degree temperature changes, it would be inherent for the control system to require higher differential temperature thresholds. These higher temperature thresholds require the heater control to add more energy than is needed, heating the water to a higher temperature than is necessary. Since the rate of loss of temperature through the heat exchanger increases as the fluid is heated higher in relationship to the ambient temperature of its environment, it would be desirable to precisely maintain small temperature thresholds. Furthermore, without the ability to maintain such small temperature differences, small drips from the hot water fixture will require the heater to cycle more often and will result in larger and unnecessary energy consumption.
Problem 5. Most commercially available fluid heaters have few diagnostic features. The user finds out he has a problem only after the heater has failed and that's usually at the most inopportune time. In worse cases, heaters are located in areas such as the attic or other location not easily inspected by the user. If the heater or its related plumbing starts to leak, the user may not know about it until substantial water damage has occurred. When a thermostat or other temperature-measuring device fails in a flow-through fluid heater, conventional systems depend on over temperature devices (high limit thermostats) to disengage power to the elements. In a high wattage flow-through heaters, the fluid may reach very dangerous levels within a shorter period of time than the response time for most of these high limit temperature devices. It is for this very reason that pressure relief valves are required in most storage tank heaters. Even so, there unfortunately are many serious injuries each year caused by fluid heaters that have ruptured as a result of an over temperature condition caused by failed temperature and pressure sensing devices.
Problem 6. Level detection circuitry has been used to verify that there is a safe level of fluid within the heat exchanger. Seitz et al in U.S. Pat. No. 5,216,743 disclose level detect circuitry that operates by conducting a small amount of current through the water at the ac line frequency. If water is at the proper level, a conductive sensor in fluid contact will deliver current through the water to a ground source also in fluid contact. This approach is particularly dependent for its reliability on the impedance of the water. Since the impedance of water varies greatly throughout the world, this approach is inherently subject to occasional unreliability. Furthermore, an electric heating element, adjacent to the water level probe, may have a corroded sheath causing electrical leakage into the water. Such a circuit may erroneously indicate insufficient water level because in an electrical flow-through water heater, electrical current from the leaky element to ground will register as a small voltage on the probe. The ac voltage will be sensed, rectified and, if it is above the water level detection threshold, i.e., of 2.2 volts, will cause the heater to shutdown due to an erroneous water level detection fault.
Problem 7. Residential water heaters are manufactured to operate at a predetermined range of set-point temperatures. Generally one can manually adjust the temperature set point. Given the diversity of hot water applications within a home or business there is a need for the user to be able to remotely adjust the set-point temperature.
Problem 8. Any flow-through water heater having sufficient power to take care of the requirements for a whole house in areas in which the ambient water can drop as low as 40 degrees Fahrenheit requires available power to provide a minimum input of 75,000 btu. An electric water heater with a capacity of at least 22,000 watts or 22 kW is required. The possible current demand from a 22 kW electric heater operating at 240 volts is approximately 92 amperes. Many homes built within the last 10 years, do not have more than 125 amperes service available to the house. Under National Electric Code Article 220-31, a method is provided to calculate the total load requirements for adding load to an existing dwelling. Such a water heater's load can be calculated at a demand factor of 40% by virtue of diversity of use and reduces the water heater's load requirements for load calculations. Nevertheless in areas where there exist large air conditioning or electric space heating loads in homes, it is very unlikely to be able to install a 22 kW heater in a home without increasing the electrical service to the home. Upgrading a homes electrical service is an expensive process and has eliminated a large section of the existing residential retrofit market for such high wattage electric flow heaters.
Other Problems. Another consideration for residential hot water use is the fact that the limitation in quantity of useable hot water from a storage tank heater in itself governs how long a family member may stay in the shower using hot water. When a flow-through water heater of sufficient capacity is used, however, the only limitation on hot water availability is its maximum flow rate capability. Given this fact it would be very desirable to be able to limit remotely and by personal code the actual time allocated to various family members for a single show.
Because water heaters are, as previously mentioned, often times located in attics or other remote locations, it would not only be desirable to have self diagnostic features in the water heater but also the ability to remotely be advised of a fault or failure.
The disadvantages of the prior art are overcome by the present invention.